The present invention generally relates to processes and apparatuses for manufacturing wafers. More particularly, the invention relates to processes and apparatuses for exfoliating the external surface of an ingot to more efficiently produce solar grade photovoltaic wafers and the like therefrom.
Conventionally, a wafer material such as monocrystalline silicon is processed into solar grade photovoltaic (“PV”) wafers by first creating a single crystalline cylindrical ingot of silicon. The ingot is created by melting high-purity semiconductor-grade wafer material in an inert chamber, such as one made of quartz. Dopant impurity atoms such as boron, phosphorus, arsenic, or antimony may be added to the molten wafer material in precise amounts (e.g., on the order of 1013 or 1016 atoms per cm3) to define the material as either a bulk n-type (negative) or p-type (positive) semiconductor, which gives the wafer material the desired electrical properties. Then, a rod-mounted seed crystal is dipped into the molten wafer material and slowly pulled upwards and rotated simultaneously to extract a preferably single-crystal cylindrical ingot. Controlling the temperature gradient, extraction rate, and rotation speed facilitates the production of a single ingot with only trace amounts of unwanted instabilities. The process is normally performed in an inert atmosphere such as argon.
Individual wafers are basically created by slicing a thin layer of the semiconductor material off from this larger ingot. Wafers may be square, rectangular or circular and are frequently used in the fabrication of integrated circuits and other micro or electronic devices, such as solar panels. In one example, circular wafers are sliced off the end of the cylindrical ingot by a diamond coated wire roughly 20 micrometers in diameter. The problem with this production method is that the diamond wire shaves a portion of the ingot into dust in a thickness equal to the diameter of the diamond coated wire. Thus, for each circular wafer created, at least 20 micrometers of wafer material is wasted as dust residue.
But, these circular wafers are not preferred for use with solar panels because square or rectangular wafers better maximize surface area exposure to sunlight energy. To make square or rectangular wafers, the stock cylindrical ingot is, instead, first squared into an elongated rectangular box shape approximately 1.5 meters long. This squaring process uses a similar conventional 20 micrometer diameter diamond coated wire. Similar to the above, portions of the exterior of the ingot are lost to dust as the diamond wire cuts through portions of the ingot to form the rectangular block. Furthermore, this squaring process requires relatively large chunks of valuable and expensive wafer material to be chopped off and thrown away to square the cylindrical ingot. From here, individual square or somewhat rectangular wafers are sliced off the end of the rectangular semiconductor block, as described above with respect to the circular wafers. While hundreds of relatively square or rectangular wafers ranging in thickness from 160 to 200 micrometers can be sliced off this rectangular semiconductor block, each wafer cut wastes an amount of wafer material equal to the width of the diamond wire cutting the semiconductor block. Another drawback in cutting wafers with a diamond-coated wire is that the saw can cause surface damage to the wafer that requires repair.
Recently, newer technologies have been developed to create additional, thinner wafers from existing wafers cut from the silicon ingot or rectangular silicon block, as described above. For example, U.S. Pat. No. 7,939,812 to Glavish et al., U.S. Pat. No. 7,982,197 to Smick et al., U.S. Pat. No. 7,989,784 to Glavish et al., and U.S. Pat. No. 8,044,374 to Ryding et al., the contents of each reference are herein incorporated by reference in their entireties, disclose a hydrogen ion implanter used to exfoliate silicon wafers to produce a thinner lamina of crystalline semiconductor material. In this respect, the ion implanter penetrates the surface of a silicon wafer to a certain depth. This penetrated layer of silicon can then be peeled back away from the silicon wafer (i.e., exfoliated)—effectively creating a thinner silicon wafer using the original silicon wafer as a workpiece. Using this exfoliation process, a silicon wafer workpiece on the order of 160-200 micrometers can be used to create 8-10 new silicon wafers having a thickness of approximately 20 micrometers, with nearly no silicon material wasted during the process. Further to this concept, U.S. Pat. Nos. 8,058,626 and 8,089,050, both to Purser et al., the contents of which are both herein incorporated by reference, disclose embodiments for creating a modified ribbon-shaped ion beam having an elongated cross-section normal to the beam direction for use in the aforementioned process for implanting ions into the surface of a substrate.
The current exfoliation processes, such as those described above, require two steps to create a sheet of exfoliated wafer material. More specifically, individual wafers are exfoliated from an ingot in one process step and then the exfoliated layer or wafer is removed from the ingot in a second process step. This two-step conventional process is costly and time consuming by virtue of to its multi-step nature. Furthermore, this conventional process produces a large number of individual exfoliated sheets of wafer material that are relatively expensive to handle and stamp into individual wafers.
Typically, conventional solar cells are manufactured from silicon produced through the Czochralski process, which can result in undesirably high oxygen content (e.g., 1018 oxygen atoms per cubic centimeter) as a result of using a crucible. Impurities in silicon wafers, such as oxygen, reduce the voltage and current capacities of the solar cell. As such, lower oxygen content silicon such as float zone silicon (“FZ silicon”) are more desirable as FZ silicon produces more efficient solar cells. FZ silicon is made in a process called vertical zone melting, wherein a polycrystalline rod of ultra-pure electronic grade silicon is passed through an RF heating coil to create a localized molten zone. A seed crystal is used at one end of the rod to start crystal ingot growth. The vertical zone melting process is carried out in an evacuated chamber or in an inert gas purge. Unlike the Czochralski process, the molten zone carries impurities such as oxygen away from the silicon ingot during growth (e.g., because most impurities are more soluble in the melt than the crystal), thereby reducing the impurity concentration within the silicon ingot. As such, FZ silicon is relatively more pure than silicon made from the Czochralski process. But, the problem with FZ silicon is that it must be cut into thicker than desired wafer sizes (e.g., on the order of 300-500 microns in thickness) because the rigid material properties prevent known methods (e.g., a diamond wire) from cutting the material any thinner. Thus, silicon wafers made from FZ silicon or the like are currently cost prohibitive due to material costs and limitations regarding the currently available minimum manufacturing thickness of the wafers.
There exists, therefore, a significant need in the art for processes and related apparatuses for more efficiently producing square and rectangular wafers from an FZ silicon stock ingot. Such processes and related apparatuses may include steps for mounting a square or rectangular FZ silicon ingot, penetrating a selected layer of the outer surface of the ingot, exfoliating away this bombarded layer of wafer material along one or more sides of the rectangular or square FZ silicon ingot, and conveying that strip of material to a press to be sliced into individual wafers, all without the waste associated with cutting or slicing the ingot into individual wafers with a diamond saw. Such processes and apparatuses may further be able to simultaneously exfoliate and remove a single continuous sheet of wafer material from the ingot. The present invention fulfills these needs and provides further related advantages.